In electrophotography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element comprising at least two layers: a charge generation layer and an electrically conductive substrate. The electrostatic latent image can be formed by a variety of means, for example, by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
Among the many different kinds of charge generation materials which have been employed in electrophotographic elements are phthalocyanine pigments such as titanyl phthalocyanine and titanyl tetrafluorophthalocyanine. Electrophotographic recording elements containing such pigments as charge-generation materials are useful in electrophotographic laser beam printers because they are capable of providing good photosensitivity in the near infrared region of the electromagnetic spectrum, that is in the range of 700-900 nm.
Charge generation layers containing titanyl phthalocyanine pigment are produced from liquid coating compositions that include the charge generation material and a solvent solution of polymeric binder. It is necessary that the phthalocyanine be in a form, that is highly photoconductive and sufficiently and stably dispersed in the coating composition to permit its being applied at a low enough concentration to form a very thin layer having high electrophotographic speed in the near infrared range.
Titanyl phthalocyanines, including titanyl fluorophthalocyanine, can assume a variety of crystalline forms, referred to as "polymorphs", that are distinguished by differing x-ray diffraction spectra. (Crystallographic characteristics discussed herein, are based upon x-ray diffraction spectra at the Bragg angle 2.theta. using CuK.alpha. x-radiation at a wavelength of 1,541 .ANG. and are .+-.0.2.degree. unless otherwise indicated. Suitable x-ray diffraction techniques are described, for example, in Engineering Solids, T. S. Hutchinson and D. C. Baird, John Wiley and Sons, Inc., 1963 and X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd Ed., John Wiley and Sons, Inc., 1974. Polymorphs of titanyl phthalocyanines can be broadly divided into low crystallinity polymorphs, also referred to as "amorphous" polymorphs; and high crystallinity polymorphs.
A variety of methods have been used to produce various forms of titanyl phthalocyanine. Different methods have commonly produced titanyl phthalocyanines having differing crystallographic characteristics, i.e, different polymorphs. U.S. Pat. No. 5,166,339, to Duff et al, presents a table of polymorphs of unsubstituted titanyl phthalocyanine (also referred to as "TiOPc" in which materials bearing multiple designations are placed in four groups. Many types of TiOPc and other phthalocyanines are discussed in Organic Photoreceptors Systems, P. M. Borsenberger and D. S. Weiss, Marcel Dekkar, Inc., New York, pp. 338-391.
Titanyl phthalocyanine preparation methods share the common characteristic of producing pigment having a small particle size. In one group of preparation methods, crude titanyl phthalocyanine is milled, generally with a particular milling media. In another group of preparations, commonly referred to as "acid-pasting", crude titanyl phthalocyanine is dissolved in an acid solution and then mixed with a large amount of a nonsolvent to precipitate the titanyl phthalocyanine product. Some preparations combine techniques or modify a previously prepared titanyl phthalocyanine.
The following are examples of such preparation methods. U.S. Pat. No. 5,153,094, to Kazmaier, teaches an acid pasting process in which crude titanyl phthalocyanine was dissolved in trihaloacetic acid and toluene and precipitated into toluene, alcohol, water, or a water-alcohol mixture. U.S. Pat. No. 5,166,339, to Duff et al, describes a process similar to Kazmaier which utilized as nonsolvents: water, alcohol, diethyl ether, ethylene glycol, dimethyl sulfoxide, dimethyl formamide, and N-methylpyrrolidone. U.S. Pat. No. 5,182,382, to Mayo et al, teaches a process for the preparation of Type X titanyl phthalocyanine by dissolving titanyl phthalocyanine in trifluoroacetic acid and methylene chloride; adding the solution to a solvent mixture of alcohol and water; and separating and washing the product. U.S. Pat. No. 5,132,197, to Iuchi et al, teaches a method in which titanyl phthalocyanine was acid pasted, treated with methanol, and milled with ether, monoterpene hydrocarbon, or liquid paraffin to produce a titanyl phthalocyanine having main peaks of the Bragg angle 2.theta. with respect to X-rays of Cu K.alpha. at 9.0, 14.2, 23.9, and 27.1 degrees (all .+-.0.2.degree.). U.S. Pat. No. 5,206,359, to Mayo et al, teaches a process in which titanyl phthalocyanine produced by acid pasting is converted to type IV titanyl phthalocyanine from Type X by treatment with halobenzene. U.S. Pat. No. 5,059,355, to Ono et al, teaches a process in which .alpha.- or .beta.-TiOPc was shaken with glass beads producing an amorphous material having no substantial peaks by X-ray diffraction. The amorphous material was stirred with heating in water and ortho-dichlorobenzene. Methanol was added after cooling. A crystalline material was produced which had a distinct peak at 27.3.degree. U.S. Pat. No. 4,882,427, to Enokida et al, teaches a material having noncrystalline titanyl phthalocyanine and pseudo-noncrystalline titanyl phthalocyanine. The pseudo-noncrystalline material could be prepared by acid pasting or acid slurrying. The noncrystalline titanyl phthalocyanine could be prepared by acid pasting or acid slurrying followed by dry or wet milling, or by mechanical milling for a long time without chemical treatment. U.S. Pat. No. 5,194,354, to Takai et al, teaches that amorphous titanyl phthalocyanine prepared by dry pulverization or acid pasting can be converted, by stirring in methanol, to a low crystalline titanyl phthalocyanine having strong peaks of the Bragg angle 28 with respect to X-rays of Cu K.alpha. at 7.2, 14.2, 24.0 and 27.2.degree. (all .+-.0.2.degree.). The low crystalline material, it was indicated, could be treated with various organic solvents to produce crystalline materials: methyl cellosolve or ethylene for material having strong peaks at 7.4, 10.9, and 17.9.degree.; propylene glycol, 1,3-butanediol, or glycerine for material having strong peaks at 7.6, 9.7, 12.7, 16.2, and 26.4.degree.; and aqueous mannitol solution for material having strong peaks at 8.5 and 10.2.degree. (all peaks .+-.0.2.degree.). U.S. Pat. Nos. 4,994,566 and 5,008,173, to Mimura et al, teach a process in which non-crystalline particles produced by: acid pasting or slurrying then mechanical grinding, mechanical grinding for a very long time, or sublimation; are treated with tetrahydrofuran to produce the a titanyl phthalocyanine having infrared absorption peaks at 1,332; 1,074; 962; and 783 cm.sup.-1 U.S. Pat. No. 5,039,586, to Itami, teaches acid pasting followed by milling in aromatic or halide with or without additional water or other solvents such as alcohols or ethers, at 20.degree.-100.degree. C. In an example, crude titanyl phthalocyanine was milled with .alpha.-chloronaphthalene or ortho-dichlorobenzene as milling medium followed by washing with acetone and methanol. The titanyl phthalocyanine produced had a first maximum intensity peak of the Bragg angle 2.theta. with respect to X-rays of Cu K.alpha. at a wavelength of 1.541 .ANG. at 27.3.degree..+-.0.2.degree. and a second maximum intensity peak at 6.8.degree..+-.0.2.degree.. This was contrasted with another titanyl phthalocyanine which was similarly milled, but not acid pasted. This material had a maximum peak at 27.3.degree..+-.0.2.degree. and a second maximum intensity peak, in the 6.degree.-8.degree. range, at 7.5.degree..+-.0.2.degree. . U.S. Pat. No. 5,055,368, to Nguyen et al, teaches a "salt-milling" procedure.
U.S. Pat. Nos. 5,238,764 and 5,238,766, both to Molaire, teach the preparation of different titanyl fluorophthalocyanine polymorphs by contacting amorphous titanyl fluorophthalocyanine with either an organic solvent having a gamma.sub.c hydrogen bonding parameter greater than 9.0 or an organic solvent having a hydrogen bonding parameter less than 8.0. It is noted that amorphous titanyl fluorophthalocyanines produced by acid-pasting and salt-milling procedures, unlike unsubstituted titanyl phthalocyanine, suffer a significant reduction in near infrared sensitivity when dispersed in a solvent such as methanol or tetrahydrofuran, which has a gamma.sub.c hydrogen bonding parameter value greater than 9.0; but do not suffer this reduction in sensitivity if first contacted with a material having a gamma.sub.c hydrogen bonding parameter of less than 8.0.
Acid pasting of titanyl fluorophthalocyanine is taught in U.S. Pat. No. 4,701,396. The crude pigment is dissolved in cold concentrated mineral acid, such as concentrated sulfuric acid, and then poured into ice water to precipitate the pigment. The cold temperature of the water reduces spattering and the like caused by mixing the acid and water and encourages more complete precipitation of the dissolved pigment.
Not surprisingly, acid pasting suffers from the shortcoming that some of the acid used remains as a contaminant in the precipitated phthalocyanine pigment. This causes a degradation of electrophotographic characteristics. The amount of residual acid can be reduced by washing. U.S. Pat. No. 4,701,396 teaches washing the acid pasted titanyl fluorophthalocyanine in boiling water, followed by hot filtration and air drying.
The present inventors have discovered surprising shortcomings in the procedures of U.S. Pat. No. 4,701,396.
It is therefore desirable to provide titanyl fluorophthalocyanine polymorphs that exhibit improved electrophotographic characteristics, and compostions of matter and electrophotographic elements including these polymorphs, and preparation methods.